The knowledge of the overall amplitude and phase distortion of a far end of a transmission channel is particularly important to adjust the equalizers placed at the ends of a transmission line, especially when regenerating devices are not provided, i.e. when an analog transmission channel is involved.
A classical method used to obtain the phase distortion is that of making a group-delay measurement.
A frequency-swept and amplitude-modulated or phase-modulated signal is sent onto the channel and the modulating signal, which is constant neither in amplitude nor in phase, is extracted from the received signal at the channel end. The amplitude variation is a function of the amplitude distortion whereas the phase variation is a function of the group delay. Phase distortion can be determined from these values by reprocessing the measurements. However, such operations, basically consisting of a series of integrations, make the measurements slower and more complicated. In addition, a compact-enough instrument, endowed with suitable bandwidth, measurement processing and display capability and easy to use outside the laboratory, is seldom found.